Surface modification of carbon fibers

ABSTRACT

A PROCESS IS PROVIDED FOR MODIFYING THE SURFACE CHARACTERISTICS OF A CARBONACEOUS FIBROUS MATERIAL (I.E. EITHER AMORPHOUS CARBON OR GRAPHITIC CARBON) AND TO THEREBY FACILITATE ENHANCED ADHESION BETWEEN THE FIBROUS MATERIAL AND A MATRIX MATERIAL. THE CARBONACEOUS FIBROUS MATERIAL IS CONTACTED FOR RELATIVELY BRIEF RESIDENCE TIME WITH AN EXCITED GAS SPECIES AT A MODERATE TEMPERATURE OF ABOUT 20 TO 325*C. GENERATED BY APPLYING HIGH FREQUENCY ELECTRICAL ENERGY IN PULSED FROM TO A GASEOUS MIXTURE COMPRISING ING AN INERT GAS AND A SURFACE MODIFICATION GAS. COMPOSITE ARTICLES OF ENHANCED INTERLAMINAR SHEAR STRENGTH MAY BE FORMED BY INCORPORATING THE FIBERS MODIFIED IN ACCORDANCE WITH THE PRESENT PROCESS IN A RESINOUS MATRIX MATERIAL.

July 10, 1973 K c. HOU 3,745,104

SURFACE MODIFICATION O F CARE ON FIBERS Filed Dec. 17, 1970 3Sheets-Sheet 2;

E i I i x 1 l N i m/l/E/vrae, KENNETH C //0u V P V :1 ii I:

July 10, 1973 K. c. HOU

I SURFACE MODIFICATION OF CARBON FIBERS 3 SheetsSheet 3 Filed Dec. 17,1970 &m. 8

SMG,

PULSE GENERATOR 13C) a United States Patent US. Cl. 204-164 19 ClaimsABSTRACT OF THE DISCLOSURE A process is provided for modifying thesurface characteristics of a carbonaceous fibrous material (i.e. eitheramorphous carbon or graphitic carbon) and to thereby facilitate enhancedadhesion between the fibrous material and a matrix material. Thecarbonaceous fibrous material is contacted for relatively briefresidence time with an excited gas species at a moderate temperature ofabout 20 to 325 C. generated by applying high frequency electricalenergy in pulsed form to a gaseous mixture comprising an inert gas and asurface modification gas. Composite articles of enhanced interlaminarshear strength may be formed by incorporating the fibers modified inaccordance with the present process in a resinous matrix material.

BACKGROUND OF THE INVENTION In the search for high performancematerials, considerable interest has been focused upon carbon fibers.The term carbon fibers is used herein in its generic sense and includesgraphite fibers as Well as amorphous carbon fibers. Graphite fibers aredefined herein as fibers which consist essentially of carbon and have apredominant X- ray diffraction pattern characteristic of graphite.Amorphous carbon fibers, on the other hand, are defined as fibers inwhich the bulk of the fiber weight can be attributed to carbon and whichexhibit an essentially amorphous X-ray diffraction pattern. Graphitefibers generally have a higher Youngs modulus than do amorphous carbonfibers and in addition are more highly electrically and thermallyconductive.

Industrial high performance materials of the future are projected tomake substantial utilization of fiber reinforced composites, and carbonfibers theoretically have among the best properties of any fiber for useas high strength reinforcement. Among these desirable properties arecorrosion and high temperature resistance, low density, high tensilestrength, and high modulus. Graphite is one of the very few knownmaterials whose tensile strength increases with temperature. Uses forcarbon fiber reinforced composites include aerospace structuralcomponents, rocket motor casings, deep-submergence vessels and ablativematerials for heat shields on re-entry vehicles.

In the prior art numerous materials have been proposed for use aspossible matrices in which carbon fibers may be incorporated to providereinforcement and produce a composite article. The matrix material whichis selected is commonly a thermosetting resinous material and iscommonly selected because of its ability to also withstand highlyelevated temperatures.

While it has been possible in the past to provide carbon fibers ofhighly desirable strength and modulus charac teristics, difiicultieshave arisen when one attempts to gain the full advantage of suchproperties in the resulting carbon fiber reinforced composite article.Such inability to capitalize upon the superior single filamentproperties of the reinforcing fiber has been traced to inadequate ad-'hesion between the fiber and the matrix in the resulting compositearticle.

Various techniques have been proposed in the past for modifying thefiber properties of a previously formed car- "ice - normally within therange of 350 C. to 850 C. (e.g. 500

to 600 C.) in an oxidizing atmosphere such as air for an appreciableperiod of time (e.g. one hour or more). Other atmospheres contemplatedfor use in the process include an oxygen rich atmosphere, pure oxygen,or an atmosphere containing an oxide of nitrogen from which free oxygenbecomes available such as nitrous oxide and nitrogen dioxide.

It is an object of the invention to provide a process for efficientlymodifying the surface characteristics of carbon fibers with nosubstantial reduction in the single filament tensile properties.

It is an object of the invention to provide a process for improving theability of carbon fibers to bond to a resinous matrix material.

It is an object of the invention to provide a process for modifying thesurface characteristics of carbon fibers which may be conductedrelatively rapidly at moderate temperatures and at atmospheric pressure.

It is another object of the invention to provide composite articlesreinforced with carbon fibers exhibiting improved interlaminar shearstrength.

These and other objects, as well as the scope, nature, and utilizationof the invention will be apparent from the following detaileddescription and appended claims.

SUMMARY OF THE INVENTION It has been found that a process for themodification of the surface characteristics of a carbonaceous fibrousmaterial containing at least about 90 percent carbon by weight comprises(a) providing in a surface modification zone at a pressure of about 1 to3 atmospheres a gaseous mixture comprising about to 99.9 percent byvolume of an inert gas and about 0.1 to 20 percent by volume of asurface modification gas selected from the group consisting of oxygen,carbon dioxide, nitric oxide, nitrous oxide, nitrogen dioxide, sulfurdioxide, water, and mixtures of the foregoing, (b) applying highfrequency electrical power in pulsed form to said gaseous mixturesufficient to establish an excited gas species within said surfacemodification zone while maintaining the temperature of said zone atabout 20 to 325 C., and (c) contacting said carbonaceous fibrousmaterial while present in said surface modification zone with saidexcited gas species until the ability of said carbonaceous fibrousmaterial to bond to a matrix material is beneficially enhanced.

The resulting carbon fibers may be incorporated in a resinous matrixmaterial to form a composite article exhibiting enhanced interlaminarshear strength.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a schemtic illustration of arepresentative apparatus arrangement for modifying the surfacecharacteristics of a carbonaceous fibrous material in accordance withthe present process.

FIGS. 1A and 1B are schematic illustrations of alter native means forcapacitively exciting the gaseous mixture in the surface modificationzone of FIG. 1.

FIG. 1C is a schematic illustration of means for inductively excitingthe gaseous mixture in the surface modification zone of FIG. 1.

FIG. 2 is a schematic illustration of a further representative apparatusarrangement for modifying the surface characteristics of a carbonaceousfibrous material in accordance with the present process wherein amercury pool surrounds the surface modification zone.

3 DESCRIPTION OF PREFERRED EMBODIMENTS The starting material The fiberswhich are surface modified in accordance with the present process arecarbonaceous and contain at least about 90 percent carbon by weight.Such carbon fibers may exhibit either an amorphous carbon or apredominantly graphitic carbon X-ray diffraction pattern. In a preferredembodiment of the process the carbonaceous fibers which undergo surfacetreatment contain at least about 95 percent carbon by weight, and atleast about 99 percent carbon by weight in a particularly preferredembodiment of the process.

The carbonaceous fibrous material may be provided as either a continuousor a discontinuous length. In a preferred embodiment of the process thecarbonaceous fibrous material is a continuous length which may be in anyone of a variety of physical configurations provided substantial accessto the fiber surface is possible during the surface modificationtreatment described hereafter. For instance, the carbonaceous fibrousmaterial may assume the configuration of a continuous length of amultifilament yarn, tape, tow, strand, cable, or similar fibrousassemblage. In a preferred embodiment of the process the carbonaceousfibrous material is one or more continuous multifilament yarn. When aplurality of multifilament yarns are surface treated simultaneously,they may be continuously passed through the heating zone while inparallel and in the form of a flat ribbon.

The carbonaceous fibrous material which is treated in the presentprocess optionally may be provided with a twist which tends to improvethe handling characteristics. For instance, a twist of about 0.1 to 5t.p.i., and preferably about 0.3 to 1.0 t.p.i., may be imparted to amultifilament yarn. Also, a false twist may be used instead of or inaddition to a real twist. Alternatively, one may select continuousbundles of fibrous material which possess essentially no twist.

The carbonaceous fibers which serve as the starting material in thepresent process may be formed in accordtnce with a variety of techniquesas will be apparent to those skilled in the art. For instance, organicpolymeric fibrous materials which are capable of undergoing thermalstabilization may be initially stabilized by treatment in an appropriateatmosphere at a moderate temperature (e.g. 200 to 400 C.), andsubsequently heated in an inert atmosphere at a more highly elevatedtemperature, e.g. 900 to 1000 C., or more, until a carbonaceous fibrousmaterial is formed. If the thermally stabilized material is heated to amaximum temperature of 2000 to 3100" C. (preferably 2400 to 3100 C.) inan inert atmosphere, substantial amounts of graphitic carbon arecommonly detected in the resulting carbon fiber, otherwise the carbonfiber will commonly exhibit an essentially amorphous X-ray diffractionpattern.

The exact temperature and atmosphere utilized during the initialstabilization of an organic polymeric fibrous material commonly varywith the composition of the precursor as will be apparent to thoseskilled in the art. During the carbonization reaction elements presentin the fibrous material other than carbon (e.g. oxygen and hydrogen) aresubstantially expelled. Suitable organic polymeric fibrous materialsfrom which the fibrous material capable of undergoing carbonization maybe derived include an acrylic polymer, a cellulosic polymer, apolyamide, a polybenzimidazole, polyvinyl alcohol, etc. As discussedhereafter, acrylic polymeric materials are particularly suited for useas precursors in the formation of carbonaceous fibrous materials.Illustrative examples of suitable cellulosic materials include thenatural and regenerated forms of cellulose, e.g. rayon. Illustrativeexamples of suitable polyamide materials include the arcmaticpolyamides, such as nylon 6T, which is formed by the condensation ofhexamethylenediamine and terephthalic acid. An illustrative example of asuitable poly- .4 benzimidazole is poly 2,2 m-phenylene 5,5-bibenzimidazole;

A fibrous acrylic polymeric material prior to stabilization may beformed primarily of recurring acrylonitrile units. For instance, theacrylic polymer should contain not less than about mol percent ofrecurring acrylonitrile units with not more than about 15 mol percent ofa monovinyl compound which is copolymerizable with acrylonitrile such asstyrene, methyl acrylate, methyl methacrylate, vinyl acetate, vinylchloride, vinylidene chloride, vinyl pyridine, and the like, or aplurality of such monovinyl compounds.

During the formation of a preferred carbonaceous fibrous material foruse in the present process multifilament bundles of an acrylic fibrousmaterial may be initially stabilized in an oxygen-containing atmosphere(i.e. preoxidized) on a continuous basis in accordance with theteachings of US. Ser. N0. 749,957, filed Aug. 5, 1968, of Dagobert E.Stutez, which is assigned to the same assignee as the present inventionand is herein incorporated by reference. More specifically, the acrylicfibrous material should be either an acrylonitrile homopolymer or anacrylonitrile copolymer which contains no more than about 5 mol percentof one or more monovinyl comonomers copolymerized with acrylonitrile. Ina particularly preferred embodiment of the process the fibrous materialis derived from an acrylonitrile homopolymer. The stabilized acrylicfibrous material which is preoxidized in an oxygen-containing atmosphereis black in appearance, commonly contains a bound oxygen content of atleast about 7 percent by weight as determined by the Unterzaucheranalysis, retains its original fibrous configuration essentially intact,and is non-burning when subjected to an ordinary match flame.

In preferred techniques for forming the starting material for thepresent process a stabilized acrylic fibrous material is carbonized andgraphitized while passing through a temperature gradient present in aheating zone in accordance with the procedures described in commonlyassigned U.S. Ser. Nos. 777,275, filed Nov. 20, 1968 of Charles M.Clarke (now abandoned); 17,780, filed Mar. 9, 1970 of Charles M. Clarke,Michael J. Ram, and John P. Riggs; and 17,832, filed Mar. 9, 1970 ofCharles M. Clarke, Michael J. Ram, and Arnold J. Rosenthal. Each ofthese disclosures is herein incorporated by reference.

In accordance with a particularly preferred carbonization andgraphitization technique a continuous length of stabilized acrylicfibrous material which is non-burning when subjected to an ordinarymatch flame and derived from an acrylic fibrous material selected fromthe group consisting of an acrylonitrile homopolymer and acrylonitrilecopolymers which contain at least about 85 mol percent of acrylonitrileunits and up to about 15 mol percent of one or more monovinyl unitscopolymerized therewith is converted to a graphitic fibrous materialwhile preserving the original fibrous configuration essentially intactWhile passing through an inert gaseous graphitization heating zonecontaining an inert gaseous atmosphere and a temperature gradient inwhich the fibrous material is raised within a period of about 20 toabout 300 seconds from about 800 C. to a temperature of about 1600 'C.to form a continuous length of carbonized fibrous material, and in whichthe carbonized fibrous material is subsequently raised from about 1600C. to a maximum temperature of at least about 2400 C. within a period ofabout 3 to 300 seconds where it is maintained for about 10 seconds toabout 200 seconds to form a continuous length of graphitic fibrousmaterial.

The equipment utilized to produce the heating zone used to produce thecarbonaceous starting material may be varied as will be apparent tothose skilled in the art. It is essential that the apparatus selected tobe capable of producing the required temperature while excluding thepresence of an oxidizing atmosphere.

In a preferred technique the continuous length of fibrous materialundergoing carbonization is heated by use of an induction furnace. Insuch a procedure the fibrous material may be passed in the direction ofits length through a hollow graphite tube or other susceptor which issituated within the windings of an induction coil. By varying the lengthof the graphite tube, the length of the induction coil, and the rate atwhich the fibrous material is passed through the graphite tube, manyapparatus arrangements capable of producing carbonization orcarbonization and graphitization may be selected. For large scaleproduction, it is of course preferred that relatively long tubes orsusceptors be used so that the fibrous material may be passed throughthe same at a more rapid rate while being carbonized or carbonized andgraphitized. The temperature gradient of a given apparatus may bedetermined by conventional optical pyrometer measurements as will beapparent to those skilled in the art. The fibrous material because ofits small mass and relatively large surface area instantaneously assumesessentially the same temperature as that of the zone through which it iscontinuously passed.

The gaseous mixture Within the surface modification zone is provided agaseous mixture comprising about 80 to 99.9 percent by volume(preferably 90 to 99 percent by volume) of an inert gas and about 0.1 to20 percent by volume (preferably l to percent by volume) of a surfacemodification gas (described hereafter).

Suitable inert gases for inclusion in the gaseous mixture includenitrogen, helium, argon, neon, krypton, and xenon, and mixtures of theforegoing. The preferred inert gases are monoatomic, e.g. helium, argon,neon, krypton, and Xenon since these tend to undergo excitation morereadily. The particularly preferred monoatomic inert gases for use inthe process are helium and argon. The relatively high current costs ofneon, krypton, and xenon militates against their selection. When presentin the gaseous mixture, the inert gas undergoes excitation uponapplication of the high frequency electrical power in pulsed form(described hereafter) and aids in the generation of an excited gasspecies of the surface modification gas. In the absence of theappreciable presence of the inert gas in the gaseous mixture, thedesired surface modification is not accomplished because of theinability to achieve the requisite degree of excitation whilemaintaining moderate surface modification conditions, e.g. temperature.

Suitable surface modification gases for use in the process are oxygen,carbon dioxide, nitric oxide, nitrous oxide, nitrogen dioxide, sulfurdioxide, and water. The preferred surface modification gases for use inthe process are oxygen and carbon dioxide.

Provided the requisite minimum concentrations of the .inert gas andsurface modification gas are present in the gaseous mixture, remainderof the gaseous mixture by volume may be composed of other gases which donot appreciably interfere with the desired surface modification.

The gaseous mixture may be conveniently provided in the surfacemodification zone at substantially atmospheric pressure, thus avoidingthe necessity to operate under reduced pressure conditions and theconcomitant disadvantages associated therewith. Alternatively, thegaseous mixture may be provided in the surface modification zone atsuperatmospheric pressures. The gaseous mixture is commonly provided inthe surface modification zone at a pressure of about 1 to 3 atmospheres.

The gaseous mixture may be premixed prior to introduction into thesurface modification zone (described hereafter), or alternatively formedin the surface modification zone upon the introduction of separate gasstreams. It is recommended that the gaseous mixture within the surfacemodification zone be either intermittently or continuously replenished(eg. by the continuous introduction of a fresh gas supply).

6 The surface treatment The modification of the surface characteristicsof the carbonaceous fibrous material is accomplished by contacting thefibrous material while present in the surface modification zone with anexcited gas species formed through the application of pulsed highfrequency electrical power to the gaseous mixture. The carbonaceousfibrous material may be statically suspended or otherwise positionedwithin the surface modification zone. In a preferred embodiment of theprocess a continuous length of the carbonaceous fibrous material iscontinuously passed, e.g. in the direction of its length, through theexcited gas species present in the surface modification zone. Forinstance, a rotating feed roll may be provided at the entrance end ofthe surface modification zone, and a rotating take-up roll may beprovided at the exit end of the surface modification zone.

The surface modification zone may be bounded by walls constructed ofeither a conductive or a non-conductive material. For instance, atubular chamber constructed of transparent glass may be convenientlyselected to define the bounds of the zone. In such an arrangement acontinuous length of carbonaceous fibrous material may be axiallysuspended therein with free access of its surface to the excited gasspecies provided.

The excited gas species required to produce the requisite surfacemodification may be formed by inductively or capacitively couplingpulsed high frequency electric power to the gaseous mixture. Acombination of inductive and capacitive coupling may also be utilized.As shown in FIG. 1 (described in detail hereafter), the gaseous mixturewithin the surface modification zone may be capacitively excited.Representative alternative apparatus arrangements wherein capacitivecoupling also may be utilized are shown in FIG. 1A, FIG. 1B, FIG. 2(described in detail hereafter). In 'FIG. 1A the pulsed high frequencyelectrical power is applied to metallic rings which are orientedperpendicularly to the axis of an elongated surface modification zoneand elfectively surround the same. In FIG. 1B the pulsed high frequencyelectrical power is applied to a pair of mercury filled tubes orientedparallel to the axis of an elongated surface modification zone andpositioned within the same. In FIG. 1C pulsed high frequency electricalpower is inductively applied to an elongated surface modification zonethrough the use of a single coil which completely surrounds the same. 7

The term pulsed electrical power or electrical power in pulsed form asused herein is defined as pulses or bursts of high frequency electricalenergy, e.g. pulsed RF energy. The power may be an AC. signal having anamplitude of about 500 v. to 10 kv. peak-to-peak and a frequency ofabout 0.5 kHz. to 2500 mHz. (preferably 1.0 kHz. to 30 mHz.). The pulsesmay be from about 0.1 microsecond to 10 milliseconds duration(preferably 10 to 1000 microseconds). The pulse repetition rate may befrom about 0.1 kHz. to 20 mHz. (preferably about 1.0 to kHz). The pulsedelectrical power may be provided in accordance with techniques known tothose skilled in the electrical arts, e.g. by gating a high frequencyoscillator or klyston on and 01f to generate bursts of high frequencyenergy. The exact dimensions of the surface modification zone willinfluence the power requirement as will be apparent to those skilled inthe art.

The high frequency electrical power in pulsed form is applied to thegaseous mixture in suflicient quantity to establish the surfacemodifying excited gas species while maintaining the temperature of thesurface modification zone at about 20 to 325 C., and preferably at about100 to 200 C. If desired, the maintenance of the desired temperature maybe aided by immersion of the surface modification zone in a lowdielectric liquid bath, such as silicon oil.

The carbonaceous fibrous material is contacted with the excited gasspecies present within the surface modification zone until its abilityto bond to a matrix material is beneficially enhanced. Unlike many priorart surface modification techniques, the residence time required in thepresent process is relatively brief. For instance, residence times ofabout 0.2 to 20 minutes may be conveniently selected, and preferablyresidence times of about 1 to 4 minutes.

The surface modification process of the present invention otfers theadvantage of altering the surface characteristics of the carbonaceousfibrous material to the substantial exclusion of adversely influencingits single filament tensile properties of the same, i.e. tensilestrength and Youngs modulus.

The theory whereby the surface of a carbonaceous fibrous material ismodified in the present process is considered complex and incapable ofsimple explanation. It is believed, however, that the resultingmodification is attributable to a combination of physical and chemicalinteractions between the excited gas species and the carbonaceousfibrous material.

The surface modification imparted to the carbonaceous fibrous materialthrough the use of the present process exhibits an appreciable lifewhich is not diminished to any substantial degree even after the passageof 30, or more days.

The surface treatment of the present process makes possible improvedadhesive bonding between the carbonaceous fibers, and a resinous matrixmaterial. Accordingly, carbon fiber reinforced composite materials whichincorporate fibers treated as heretofore described exhibit enhancedshear strength, compressive strength, etc. The resinous matrix, materialemployed in the formation of such composite materials is commonly apolar thermosetting resin such as an epoxy, a polyimide, a polyester, aphenolic, etc. The carbonaceous fibrous material is commonly provided insuch resulting composite materials in either an aligned or randomfashion in a concentration of about 20 to 70 percent by volume.

A representative apparatus arrangement for carrying out the surfacemodification process of the invention is illustrated in FIG. 1. Withreference to FIG. 1, the power unit includes a conventional variableD.C. power supply 2, a conventional pulse generator 4 having a variablepulse repetition rate and a variable pulse width, a conventional signalamplifier 6, and a variable frequency oscillator 8. The output signalfrom the pulse generator 4 is applied to the oscillator 8 by way of thesignal amplifier 6. Both a variable positive D.C. voltage and a fixednegative bias voltage from the power supply 2 are applied to theoscillator 8.

The power supply 2 may be any conventional variable D.C. power supply,e.g. a Kepco Model 615B, -600 volt and negative 150 volt power supply.The pulse generator 4 may be any conventional pulse generator of' '1.0kHz., and which is capable of being gated or pulsed on and ofi toprovide bursts of high frequency energy. In a preferred operation of thepower unit this is accom plished by cutting off the oscillator byapplying a negative 150 volt bias to the control grid of an oscillatortube (not shown) by way of an input terminal 10 and by periodicallyapplying positive pulses to the input terminal 10 and thus the controlgrid of sufficient amplitude to drive the oscillator tube intoconduction.

In operation, the pulse generator 4 generates a series of negative goingpulses, the pulse repetition rate and/or the pulse width of which may bevaried to thereby vary the reoccurrence rate and/or the duration of thepulses. The signal from the pulse generator 4 is amplified and invertedby the amplifier 6 and the positive pulses from the amplifier 6 areapplied to the oscillator 8. In the absence of a pulse from theamplifier 6, the oscillator 8 is cut off and does not provide an outputsignal. However, when a pulse from the pulse generator 4 is applied tothe oscillator 8 by way of the amplifier 6, the oscillator 8 breaks intohigh frequency oscillations and provides an output signal for theduration of the applied pulse. The resultant pulsed high frequencysignal may be coupled to the surface modification zone 20 through aconventional high frequency step-up coil 12, the primary winding ofwhich may be utilized for both signal coupling and as a portion of theoscillator tank circuit. Lead 14 connects the coil 12 to coaxialelectrode 21. Coaxial electrode 21 consists of a 10 inch length ofcopper tubing having an outer diameter of /2 inch and an inner diameterof inch. Situated in series with coaxial electrode 21 is a like coaxialelectrode 22.

The amplitude of the output signal from the oscillator 8 may be variedby varying the voltage directly applied to the oscillator 8 from thepower supply 2. The frequency of the output signal from the oscillator 8may, of course, be varied in any suitable conventional manner, e.g. byvarying the reactive value of an electrical component in a tank circuit(not shown). In addition, the relationship between the on time and theoff time of the output signal and the duration of the pulses of highfrequency energy may be varied by adjusting the pulse repetition rateand/or width of the output pulses from the pulse generator 4. The pulseunit is thus capable of supplying brusts of electrical energy of avariable high frequency, the bursts occuring at a selectable burstrepetition rate and having a variable burst width or duration.

Another representative pulsing unit which may be used to provide thepulsed high frequency signal to excite the gas mixture in the surfacemodification zone is a Lepel Model No. T-53 high frequency power unitcapable of delivering up to a 10 kv. signal at a frequency of up to 30mHz. pulsed by a grid pulse modulator Model 1414 available from PulseTronics Engineering Co.

By providing a pulsed frequency signal as described above, excessiveheat buildup within the surface modification zone 20 may be preventedthrough variation of the pulse repetition rate, the pulse width orduration, or both of these parameters. The heat generated within thesurface modification zone during the application of pulsed highfrequency signal is allowed to dissipate to a great extent during theolf period of the oscillator, i.e. between pulses of high frequencyenergy.

Since the signal amplitude, frequency, duration and repetition raterequired for carrying out the process depend upon the diameter andlength of the surface modification zone, such parameters may varywidely. The temperature inside the surface modification zone 20 may besensed by a thermocouple 23 and a visual temperature indication may beprovided at meter 25. The temperature within the zone 20 may thus beeasily regulated by visually monitoring the meter 25 and adjusting thepulse repetion rate and/or the pulse width of the high frequency signal.The intensity of the excitation is controlled by the amplitude andduration of the pulses, the pulse repetition rate, the space gap betweenthe electrodes, and the total length of the surface modification zone.

With a surface modification zone or chamber 20 of approximately 22inches in length and 5 inch in diameter, the process may be convenientlypracticed utilizing a pulsed high frequency output signal from theoscillator 8 in the radio frequency range above 1.0 kHz., theparticularly preferred range being from 1.0 kHz. to 30 mHz. The signalmay be pulsed at a repetition rate of from about 1.0 to about 1000 kHz.(10 to 100 kHz. being preferred) while the pulse width may be from 0.1to 1000 microseconds, (1.0 to 500 microseconds being preferred). Theamplitude of the pulsed high frequency signal may be from 500 v. to 10kv. (1 to 5 kv. being preferred).

The following examples are given as specific illustrations of theprocess of the invention. It should be understood, however, that theinvention is not limited to the specific details set forth in theexamples.

EXAMPLE I Reference is made to the apparatus of FIG. 1.

A high strength-high modulus continuous filament carbonaceous yarnderived from an acrylonitrile homopolymer in accordance with theprocedures described in US. Ser. Nos. 749,957, filed Aug. 5, 1968, and777,275, filed Nov. 20, 1968 (now abandoned) was selected as thestarting material. The yarn consisted of a 1600 fil bundle having atotal denier of about 1000, had a carbon content in excess of 99 percentby weight, exhibited a predominantly graphitic X-ray diffractionpattern, a single filament tenacity of about 13 grams per denier and asingle filament Youngs modulus of about 50 million p.s.i.

The carbonaceous yarn 24 was unwound from rotating feed roll 26 intoneck 27, around pulley 28, through surface modification zone 20 viaannular guides 30 and 32, around pulley 34, and was ultimately taken upupon rotating uptake roll 36. The carbonaceous yarn 24 passed throughsurface modification zone 20 while axially suspended therein at a rateof 12 inches per minute.

The surface modification zone 20 was defined by tubular glass of about Ainch diameter and about 22 inches in length. Oxygen was continuouslyintroduced as the surface modification gas via inlet tubes 38 and 40.Helium was introduced as the inert gas via inlet tubes 44 and 40 at arate of 3000 cc. per minute. Oxygen was present in the gaseous mixturein a concentration of about 0.5 percent by weight. Oif gases were exitedvia exit tube 42. A 3000 v. peak-to-peak A.C. signal having a frequencyof 13.56 mHz. was applied to coaxial electrode 21 in pulses of 500microseconds duration at a p.r.r. (pulse repetition rate) of 100 kHz. Anexcited gas species was established throughout the length of the surfacemodification zone 20. The yarn 24 was in contact with the excited gasspecies for a residence time of about 2 minutes. Throughout the surfacemodification treatment the temperature within zone 20 was maintained atapproximately 200 C. as measured by thermocouple 23 and indicated onmeter 25. The yarn following surface treatment exhibited a singlefilament tenacity of 13 grams per denier, and a single filament Youngsmodulus of 50 million p.s.i.

A composite article was next formed employing the surface modified yarnsample as a reinforcing medium in a resinous matrix. The compositearticle was a rectangular bar consisting of about 65 percent by volumeof the yarn and having dimensions of inch x A inch x 5 inches. Thecomposite article was formed by impregnation of the yarn in a liquidepoxy resin-hardener mixture at 50 C. followed by unidirectional layupof the required quantity of the impregnated yarn in a steel mold and compression molding of the layup for 2 hours at 93 C., and 2.5 hours at 200C. in a heated platen press at about 100 p.s.i. pressure. The mold wascooled slowly to room temperature, and the composite article was removedfrom the mold cavity and cut to size for testing. The resinous matrixmaterial used in the formation of the composite article was provided asa solventless system which contained 100 parts by weight epoxy resin and88 parts by weight of anhydride curing agent.

The composite article Was found to exhibit a horizontal interlaminarshear strength of 7000 p.s.i.

The horizontal interlaminar shear strengths reported herein weredetermined by short beam testing of the carbon fiber reinforcedcomposite according to the procedure 10 of ASTM D2344-65T as modifiedfor straight bar testing at a 4:1 span to depth ratio.

For comparative purposes a composite article was formed as heretoforedescribed employing an identical carbonaceous yarn without subjectingthe same to any form of surface modification. The average horizontalinterlaminar shear strength of the composite article was only 3600p.s.i.

EXAMPLE II Example I is repeated with the exception that carbon dioxideis substituted for oxygen as the surface modification gas.

Substantially similar results are achieved.

EXAMPLE III dioxide is substituted for oxygen as the surfacemodification gas.

Substantially similar results are achieved.

EXAMPLE V Example I is repeated with the exception that the continuousfilament carbonaceous yarn undergoing surface modification was derivedfrom a cellulosic precursor and was commercially available from theUnion Carbide Company under the designation of Thornel 50. The yarn hada carbon content in excess of 99 percent by weight and exhibited apredominantly graphitic X-ray diffraction pattern. The yarn consisted ofa 2 ply 1440 fil bundle having a total denier of about 700.

When incorporated in a composite article (as described), the surfacetreated yarn exhibited a horizontal interlaminar shear strength of 7000p.s.i. When the untreated yarn was employed in the formation of acomposite article, a horizontal interlaminar shear strength of only 3600p.s.i. was exhibited.

EXAMPLE VI Reference is made to the apparatus of FIG. 2 in which likenumerals designate similar components to those previously described inconnection with FIG. 1.

A carbonaceous multi-filament yarn 24 identical to that employed inExample I is continuously unwound from feed roll 29 and passed throughmercury seal 31 supplied from reservoir 33.

The carbonaceous yarn is passed through surface modification zone 20 ata rate of 16 inches per minute. Oxygen is introduced as the surfacemodification gas via inlet tubes 38 and 40. Helium is introduced as theinert gas via inlet tubes 44 and 40 at a rate of 2000 cc. per minute.Oxygen was present in the gaseous mixture in a concentration of about 1percent by weight.

The gaseous mixture in the surface modification zone 20 was excited bymeans of the capacitance between mercury jacket 17 encircling theelectrically grounded carbonaceous yarn.

A Pulse Tronics generator is used to control a Lepel Model T-5-3 highfrequency signal generator to provide a 3000 v. peak-to-peak A.C. signalat a frequency of 10 m'Hz. in pulses of 500 microseconds duration at ap.r.r. of 10 kHz. An excited gas species is established throughout thelength of the surface modification zone 20. The yarn 24 is in contactwith the excited gas species for a residence time of about 1 minute.Throughout the surface modification treatment the temperature withinzone 20 is maintained at approximately 250 C. as measured bythermocouple 23 and indicated on meter 25.

Substantially similar surface modification results are achieved.

The nature, scope, utility, and effectiveness of the present inventionhave been described and specifically exemplitied in the foregoingspecification. However, it should be understood that these examples arenot intended to be limiting and that the scope of the invention to beprotected is particularly pointed out in the appended claims.

I claim:

1. A process for the modification of the surface characteristics of acarbonaceous fibrous material containing at least about 90 percentcarbon by weight comprising:

(a) providing in a surface modification zone at a pressure of about 1 to3 atmospheres a gaseous mixture comprising about 80 to 99.9 percent byvolume of an inert gas and about 0.1 to 20 percent by volume of asurface modification gas selected from the group consisting of oxygen,carbon dioxide, nitric oxide, nitrous oxide, nitrogen dioxide, sulfurdioxide, water, and mixtures of the foregoing,

(b) applying high frequency electrical power in pulsed form to saidgaseous mixture sufiicient to establish an excited gas species withinsaid surface modification zone while maintaining the temperature of saidzone at about 20 to 325 C., and

(c) contacting said carbonaceous fibrous material while present in saidsurface modification zone with said excited gas species until theability of said carbonaceous fibrous material to bond to a matrixmaterial is beneficially enhanced.

2. A process according to claim 1 wherein said carbonaceous fibrousmaterial includes a substantial quantity of graphitic carbon.

3. A process according to claim 1 wherein said inert gas is a monoatomicgas.

4. A process according to claim 3 wherein said monoatomic inert gas isselected from the group consisting of argon, helium, and mixtures of theforegoing.

5. A process according to claim 1 wherein said surface modification gasis oxygen.

6. A process according to claim 1 wherein said surface modification gasis carbon dioxide.

7. A process according to claim 1 wherein said surface modification gasis nitrogen dioxide.

8. A process according to claim 1 wherein said gaseous mixture isprovided at substantially atmospheric pressure.

9. A process according to claim 1 wherein said electrical power inpulsed form applied to said gaseous mixture is of the radio frequencyrange from about 1.0 kHz. to 30 mHz.

10. A process for the modification of the surface characteristics of acarbonaceous fibrous material containing at least about 90 percentcarbon by weight and exhibiting a predominantly graphitic X-raydiffraction pattern comprising:

(a) providing in a surface modification zone at a pressure of about 1 to3 atmospheres a gaseous mixture comprising about to 99.9 percent byvolume of an inert gas selected from the group consisting of argon,helium, and mixtures of the foregoing and about 0.1 to 20 percent byvolume of a surface modification gas selected from the group consistingof oxygen, carbon dioxide, nitric oxide, nitrous oxide, nitrogendioxide, sulfur dioxide, water, and mixtures of the foregoing,

(b) applying an AC. signal having an amplitude of from about 500 v. to10 kv. peak-to-peak and a frequency of from about 0.5 kHz. to 2500 mHz.to said gaseous mixture in pulses of from about 0.1 microsecond to 10milliseconds duration at a pulse repetition rate of about 0.1 kHz. to 20mHz. sufficient to form an excited gas species Within said surfacemodification zone while maintaining the temperature in said zone atabout 20 to 325 C., and

(c) continuously passing a continuous length of said carbonaceousfibrous material through said excited gas species of said surfacemodification zone for aresidence time of about 0.2 to 20 minutes therebybeneficially enhancing the ability of said carbonaceous fibrous materialto bond to a matrix material.

11. A process according to claim 10 wherein said carbonaceous fibrousmaterial contains at least about 99 percent carbon by weight.

12. A process according to claim 10 wherein said inert gas is argon.

13. A process according to claim 10 wherein said inert gas is helium.

14. A process according to claim 10 wherein said surface modificationgas is oxygen.

15. A process according to claim 10 wherein said surface modificationgas is carbon dioxide.

16. A process according to claim 10 wherein said surface modificationgas is nitrogen dioxide.

17. A process according to claim 10 wherein said gaseous mixture isprovided at substantially atmospheric pressure.

18. A process according to claim 10 wherein said A.C. signal has anamplitude from about 500 v. to 10 kv., has a frequency of about 1.0 kHz.to 30 mHz., and is pulsed at a pulse repetition rate of about 1 to 100kHz., and pulse duration of about 10 to 1000 microseconds.

19. A process according to claim 10 wherein said inert gas and saidsurface modification gas are continuously introduced into said surfacemodification zone.

References Cited UNITED STATES PATENTS 3,205,162 9/1965 MacLean 2043l23,255,099 6/1966 Wolinski 204169' 3,376,208 4/1968 Wood 204168 3,274,0889/ 1966 Wolinski 204- 3,399,252 8/1968 Hough et al. 23209.1 X 3,407,13010/ 1968 Hailstone 204-165 3,476,703 11/1969 Wadsworth et al. 260373,607,063 9/1971 Douglas 23209.1

FREDERICK C. EDMUNDSON, Primary Examiner US. Cl. X.R.

